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Altera_Forum
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18 years ago

Cyclone III AS configuration bizarreness

I am using a cyclone III (EP3C25F324C8N) with an EPCS16 configuration device.

I am programming the configuration device through the FPGA with the serial flash loader.

The problem is that the Cyclone III device will not configure itself from the EPCS16. It is stuck in an endless loop of retries UNTIL I MOMENTARILY SHORT THE nSTATUS PIN TO 3.3 VOLTS!!!! (...I discovered this by accident, no grounding the pin doesn't do it.)

As soon as I do that the device configures and runs my test program (blinks LEDs). I also tried driving it high momentarily with an 8051 to no avail.

Any ideas?

Thanks,

Rob

UPDATE: It also configured when I touched CONF_DONE with a probe. Upon further investigation I found that adding a 100 pF capacitor from CONF_DONE to GND fixes this problem ...no idea why.

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  • Altera_Forum's avatar
    Altera_Forum
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    14 years ago

    You possibly need an active probe to see the CLK signal undistorbed, The final cause for configuration problems related to JTAG signal quaility is often a non-monotic, ringing CLK edge, which is seen as double clock edge by the JTAG logic. A small capacitive load like the 10 or 12 pF of a standard passive 10:1 oscilloscope probe can be already sufficient to fix the problem.

    Placing a small capacitor, e.g. 15 or 22 pF between TCK and ground near the FPGA would be my first try to solve issue.